Manifestations of Nanomaterials in Development of Advanced...

Manifestations of Nanomaterialsin Development of Advanced Sensorsfor Defense Applications

Rohini Kitture and Sangeeta Kale

ContentsIntroduction to the Advanced Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2The Strategic Requirements of Advanced Sensors in Defense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Research Overview in Sensor Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Low-Frequency Detections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7High-Frequency Detections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Extremely Low (ppm/ ppb) Concentration Detections for CBW Diagnostics . . . . . . . . . . . . . . 15Low Electric and Magnetic Field Detections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Challenges in Current Technologies and the Route Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

AbstractIn recent times, the global science and technology is dominated by research in thenanotechnology domain, especially to explore novel materials with exotic prop-erties, which are attributed to their nano-size regimes. Typically explored exam-ples are metals (gold, silver, copper, etc.), organic and inorganic materials (metaloxides, polymers), carbon (graphene, CNTs, etc.), and so on, typically, in theirpure and composites forms. The polymers are playing a vital role in this domain,to make the polymer-based nanocomposites, which are used for different

R. KittureDepartment of Applied Physics, Defence Institute of Advanced Technology (Deemed University),Girinagar, Pune, India

SpringerNature Technology and Publishing Solutions,Magarpatta City, Pune, Indiae-mail: [email protected]; [email protected]

S. Kale (*)Department of Applied Physics, Defence Institute of Advanced Technology (Deemed University),Girinagar, Pune, Indiae-mail: [email protected]

© Springer Nature Switzerland AG 2020Y. Mahajan, R. Johnson (eds.), Handbook of Advanced Ceramics and Composites,https://doi.org/10.1007/978-3-319-73255-8_2-2

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applications in textiles, pharmaceutical, chemical, instrumentation, aerospace,aeronautical, and mechanical domains of engineering. However, one particulardomain, which has sought the maximum attention of these nanomaterials, is thesensors. Sensors are an integral part of any instrumentation, mechanical assembly,automobile engineering, heavy engineering, and drug delivery vehicles or innational surveillance gadgets or in any electromagnetic application unit, such asantennas and communication electronics. A need for smart, miniaturized,extremely sensitive, selective, and accurate sensor is always on anvil.

This chapter starts with a brief outline on the progress of science and technol-ogy, typically in the domain of sensors, for low-field and low-frequency (electricand magnetic fields and ultra-low-frequency signals) detections and chemical-biological hazardous environment detections. Various approaches for sensing,used in the authors’ laboratory, would be elaborated, namely, the radio-frequencysensing approach, optical fiber approach, metamaterial approach, and conven-tional resistive approach. The relationships of the obtained properties would beassociated with the physics and chemistry at nano-level and their energy dynam-ics for sensing a particular physical parameter. The chapter will be closely relatedto defense applications, such as chemical and biological warfare (CBW) diag-nostics and hazardous environmental detections, and electromagnetic shieldingapplications, along with low-frequency detections for sonar technologyℕ.

KeywordsNanomaterials · Nanocomposites · Electromagnetic shielding · Sensors · Opticalfibers

Introduction to the Advanced Sensors

Since the introduction of man-made thermostats in the early 1880s, sensors havebeen widely used by mankind. In general, a device that measures and converts thephysical phenomenon (temperature, displacement, force, composition, field, etc.) toproduce proportional output signal in various readable form (electrical, mechanical,magnetic, optical, etc.) can be called as sensor. This is sometimes confused withtransducers, which, in general, are known to convert one form of energy intothe other. Sensors respond to the exposed physical phenomenon or the changes inthe phenomenon to give a measurable equivalent. With the recent advances inscience and technology, most of the sensors are integrated with electronic circuitsto record and communicate the detailed reading of the measurands [1]. As describedin [1], while a sensor device produces/converts output signal due to external stimuli,the integration of this device with assorted signal processing hardware (analog ordigital) and communication system can form a complete sensor system.

Sensors can be classified in various ways. One of the simplest classifications isactive and passive sensors; the former requires external power input/signal, whereasthe latter does not require such external signal. The other classification is basedon the type of detection, viz., chemical, electrical, etc., or on the conversion

2 R. Kitture and S. Kale

phenomenon in the sensor, like electrochemical, thermoelectric, etc. Based on theway of contact between the sensor and the measurand/analyte, sensors can beclassified as contact sensors (strain gauges, conventional thermometers, etc.) andnon-contact sensors (e.g., infrared sensors, optical sensors, magnetic sensors, etc.).Depending upon the measurands, sensors can be broadly classified [2] as follows(Fig. 1).

Sensors are one domain of technology which has been around for decades now,because of their applications not only in the industrial sector but also in areas such asbiomedicine, healthcare, entertainment, fitness, and so on. As the industrial processindustry has improved from simple, semiautomatic machines to complicated,completely automatic, intelligent, and adaptive control systems, sensors have seena tremendous improvement from simple passive transducers to miniaturized smartsensors. A simple pressure transducer has been now converted into smart sensorwhich helps to operate an automated conveyer belt system. Concepts of automation,robotics, and artificial intelligence have increased the scope of sensors from anindividual component (device) to an array of devices working together and remotely(wirelessly), controlled by a software (or hardware). Wireless sensor networks findhuge applications in this new technological era, wherein not only the sensors havebeen extremely high-end but so are the networks which connect them and coordinatethem to do a certain activity following a pre-defined flowchart. Huge heavy engi-neering machinery, automobile sector, manufacturing industries, chemical indus-tries, pharmaceutical industries, entertainment electronics, textile industries,cosmetic industries, energy sectors, all require extremely well-equipped machinery,which have to use large number of sensors. These not only have to be accurate,highly sensitive, and repeatable; but they also have to be cheaper and miniaturized.Sensors have seen a progress from primitive resistive-type operating processes toimproved and smart sensing.

Many factors have been instrumental in boosting the phenomenal growthof sensors technology. First is the need, second is the invention of new materials,and third is the improvement of support electronics. The first point, which is the

•Wave•Amplitude•Spectrum

Optical

•Velocity•Force•Acceleration•Roughness

Mechanical

•Current•Electric Field•Conductivity

Electrical

•Wave ampliture•Wave velocity•Low--frequency,

high-frequency waves

Acoustic

•Temperature•Heat flux

Thermal

•Components•Gas concentra�on•Liquid concentration•Adultera�on

Chemical

•Energy•Intensity

Radiation

•Biomass

Biological

•Magnetic Field•Magnetic Flux

Magnetic

Fig. 1 Sensor classification scheme based on the type of measurand [2]

Manifestations of Nanomaterials in Development of Advanced Sensors for. . . 3

“need,” is quite normal and is responsible for growth of any technological domain.As any industry is setup and a product comes in the market, the competition and thedemand always keep the technologists on their toes and ensure that the product isbetter, is efficient, and is also cheap. This enforces the manufacturing industries todevelop high-end instrumentation, and therefore the sensors, which are embedded inthese machineries, also get better. Secondly, the materials science continues to do itsprogress with a fast pace, inventing new materials, with advanced property space.The past six to seven decades have seen phenomenal growth in materials science,right from simple metals and insulators to semiconductors to exotic materials likesuperconductors, perovskite systems, manganites, and optoelectronic materials(to name a few). Due to the evolution of different materials, in different forms(such as alloys, compounds, composites, and so on), the sensors domain has alsowidened to give better materials for similar kind of sensing. Embedded electronics,robotics, high-end interfacing tools, and software-driven machineries have beenresponsible to offer extremely good interfacing facilities to the sensors. This is thethird reason for improvement in sensor technology, i.e., “advanced electronics.”A simple coaxial cable interface of the sensor has been upgraded to the opticalinterface, wherein a small sensor signal is conditioned and made appropriate to drivehuge motors and turbines with extremely fine precision, via a sophisticated computerinterface. The electronic interfacing circuitries have become faster, and hence thesensing also has to be done with better speed and with minimal errors. Repeatability,sensitivity, accuracy, recovery, and everything have improved in the past fewdecades. Though there is a huge domain of sensor instrumentation which hasimproved the sensor technology, in this chapter, we will mostly cover the materialsaspect, which has fundamentally changed the quality of sensing.

As is said, any technological advances come to the defense sector first, and then itis taken up in the civilian market. Defense technologies are always much advancedas compared to any consumer market technologies, merely because they are relatedto the security of any country. Therefore, it is always said that any technology isdeveloped in the defense laboratories first, before it comes to civilian markets andindustrial production. Surveillance systems, arms and armament manufacturing,explosive manufacturing, healthcare sector, small-to-big missile launchers, tanktechnologies, air vehicles, guided missiles, robotics, and everything require thetechnology and industrial setups which will have hundreds of sensors implanted todo a scheduled job. Hence sensors find huge applications in defense technologies.

Miniaturization and adaptive technologies have been the mother of any progressand research. As the industrial demands increased, the type of materials used forsensing also became multifaceted. Optoelectronic materials, electrochromic mate-rials, photovoltaic materials, luminescent materials, fluorescent materials, magneto-optic materials, spintronic materials, magneto-resistive materials, and so onwere discovered and applied in various sensor applications. Fundamental materialsproperties were explored and used to yield precise sensing. Either the propertieswere of the original materials found in nature, or the materials were engineeredto give a desired property. Natural materials include quartz, silica, iron, nickel,copper, aluminum, and so on, which show property manipulation when subjectedto specific stimuli (pressure, temperature, voltage, movement, etc.). Specially


synthesized materials include manganites, diluted magnetic semiconductors, andsuperconductors, which are artificially made using materials processing techniques.There is one more category of materials, which has caught the attention ofresearchers in recent times, and these are “metamaterials.” These are completelyartificially engineered materials, and their arrangements are responsible for theproperties they exhibit, rather than the type of atomic/molecular structure theypossess (materials themselves).

As mentioned above, sensors are of various types. They measure one of multiplephysical/chemical parameters right from displacement, motion, direction, pressure,stress, strain, temperature, speed, light intensity, frequency, electric/magnetic field,concentration and nature of chemical species, and so on. A sensor can be realized, bymeasuring the temporary changes in the basic/intrinsic properties of the material, inthe form of electrical parameter. The intrinsic properties of the materials, whichtinker the sensor operation, include their atomic arrangements, stoichiometry, phasetransition, energy gap, luminescence, phosphorescence, molecular vibrational fre-quencies, atomic disorders, lattice matching (or mismatching), and so on. As theseproperties change, when a stimulus is applied to them, it is said that the material has“responded” to the sensing parameter. The “response” is exhibited in the form ofchange in resistance of the sensing material, or the voltage generated (or altered) orchanges in current flow, or modification in the frequency (if the material is frequencyselective), quenching (or enhancing), luminescence (or phosphorescence), and soon. These are calibrated in the form of their sensing action, per unit change in theparameter which has to be sensed. Potentiometric sensors, impedance sensors,frequency sensors, piezoelectric sensors, and luminescent sensors are few examples,which have been developed and used in sensor electronics market.

In this chapter, we concentrate on such novel materials which have been inventedand developed to yield high-end sensors. We discuss the property space of thesematerials and unleash their potential in the upcoming sensor technology. We limit thediscussion to defense applications, in terms of their requirements, and look into thefuture scope and challenges envisaged.

The Strategic Requirements of Advanced Sensors in Defense

Security of any country and military personnel increasingly rely on intelligent sensortechnology for surveillance and electronic intelligence. Millions of people enter intoa country through various routes and passages to either do their living or otherwise.Similarly, there is a constant danger against known and unknown enemies at theborder of any country, and therefore, a strong base of Army, Navy, and Air Force ismade ready and well equipped to guard these borders. As the terrorists become moreand more powerful and well equipped with advanced instrumentation, the country’ssecurity forces should also be equipped with equivalent (if not superior) surveillanceinstrumentation and combat equipments). Sharp and very focused weapons, long-range missiles and weaponry, smart surveillance sensors, and extremely efficienthuman force are required, along with very good communication equipments. Sim-ilarly, technology to handle the security of the men in uniform is also utmost


important. Smart bandages, camouflage technique for tents and uniforms, drugdelivery systems, tissue engineering, and communication tools are required to ensurethe safety and security of the people who are actually at the war front or who aredoing constant surveillance at the borders of country, be it air, water, or land[3]. It is the prime job of any defense sector of any country to guard their countryand fellow countrymen against the people and technologies which enter theircountry or intend to create damage in a country and have illegitimate status andintentions. Along with the manpower (people in uniform), it is extremely importantto equip them with advanced instrumentation, so that they can combat the enemy andalso protect themselves. All the scientific and technological communities strive hardto make significant strides in technology, particularly in the vital areas of intelligenceand surveillance.

“Being successful in detecting and classifying harmful threats helps ensure ourphysical well-being. The main and most challenging task is to detect and classifydangerous chemicals, materials, explosives, and people that may be threatening. Thechallenge is not only to correctly detect threats from a safe distance and ahead oftime, but to do so very quickly in order to limit the impact.” It is through theapplication of today’s advanced radars, sensors, cameras, and software that thisvery real challenge should be met, Partynski suggests [3], rather than by buildingfences or walls. Defense technologies, which apply to critical infrastructure, airports,seaports, land borders, ports of entry, and inland waterway and coastal borders,gather critical information to ensure that the defense technologies are aptly used,whenever required and wherever needed. These are the areas where high-end sensorsare required, which should be implanted, monitored, and acted upon. Technologysolutions are needed that will reduce operator workload and enable one person tomanage many surveillance systems efficiently and effectively. It is at this juncturethat smart sensors enter. Smart sensors are the goal for our future deployments ofintelligence and surveillance systems. Automated systems that detect, track, andclassify threats without radar experts, or even people continuously staring at cameravideo, will allow any country to deploy a nationwide system which would be secure,having low false alarms, and attract armed-forces attention only when action isneeded.

In addition to providing critical information on potential threats, intelligence andsurveillance systems to ensure a safety cover for people in uniform (and alsocivilians) are also necessary. Having such technologies broadly deployed will helpfind people in trouble by locating them with advanced radars, sensors, cameras, andsoftware or via their cell phones or radios. Smart sensors would allow us todetermine if activities are not normal and warrant someone to take a closer lookand possibly assist. Man-wearable sensor-embedded uniforms (or suits) are on anvil,which would help persons of interest identify themselves, when they are in panic.A quick response mechanism can be then built, when such an alarm is raised byany individual or a group of people. Video capture and display equipments,graphics capabilities, and global positioning systems are additional features requiredover a conventional smart sensing material, which will do a job with high precisionto stimulate an action plan. Man-wearable 2D/3D graphics and video-intensive


deployed unmanned aerial vehicle/unmanned ground vehicle (UAV/UGV) control,mission planning, biometric, computer-assisted maintenance, and command, con-trol, communications, computer, intelligence, and surveillance are few applicationsenvisaged in recent times. Various defense laboratories in India are tirelessly work-ing to develop such sensors and surveillance equipments. Many of these laboratoriesare involved in developing small and big arms, guided missiles, propellant systems,UAVs, radar technologies, and tanks and submarines.

All these technologies have evolved (or are evolving) and developed over andabove the basic building block of a sensor, and that is the “material.” No sensor, withwhatever advanced electronics and computing capability, can be realized without thebasic ingredient, “the material used for sensing.” In the coming paragraphs, let us gothrough the progress profile of various fascinating materials, on which manyresearch laboratories are working to envisage advanced sensors.

Research Overview in Sensor Development

Low-Frequency Detections

Low acoustic frequency signals, especially those under the water (refer to Fig. 2),have been of immense interest and research for several decades, typically for defenseapplications. In an acoustic transducer, the mechanical energy of sound waves getsconverted into electric energy (as in hydrophones, microphones, etc.), or electricenergy gets converted into sound waves (as in transmitters, loudspeakers, etc.).Piezoelectric materials, having the ability to generate an electrical charge in responseto a mechanical stress and vice versa, have been favored for acoustic transducers.For example, piezoelectric ceramics, single crystals, composites, and polymers havebeen widely used for over a century in underwater acoustic transducers. The first use

Fig. 2 Underwater frequency spectrum and the categorization of various frequency ranges


of piezoelectric material in underwater acoustic applications was reported in 1917 bya French physicist Paul Langevin, wherein quartz was used as the piezoelectricmaterial in developing an underwater acoustic transducer useful in submarinesdetection. Since then, there has been extensive research in the field [4, 5].

Low-frequency sonar waves which traverse under the water for submarine sur-veillance are extremely difficult to detect. This is due to the large impedance offeredby the water medium and also due to the noise generated due to other water bodies.Ocean surface waves, internal gravity waves, temperature, salinity, and scatteringalso have a major impact on the recorded signals. Similarly, for Navy applications,magnetic de-gauzing is also another issue. This is required to nullify the magneticfield accumulated on the ship due to the travelling in the sea for longer durations oftime. Magnetic field measurements are required to estimate the life (age) of the shipon the sea. Various passive as well as active surveillance and security systemshave been employed to detect such low-frequency signals, typically for aquatic lifemonitoring, sonar signal detections, undersea earthquake/volcano detection, andsubmarine detection. Due to poor signal-to-noise ratio, unstable shape and ampli-tude, and short-duration nature of the signals, this domain of sensor development hasproved to be rather challenging.

Though there would be many approaches for underwater surveillance, hydro-phone is one of the best ones, which has its own limitations on low-frequencydetections. Optical fiber-based approach is recently envisaged and is very good,especially from the defense applications perspective. Owing to the low electriclosses and high coupling coefficient, lead zirconate titanate (PZT) ceramics wereone of the favorite piezoelectric materials widely used for sonar application. Wher-ever electronic sensors fail due to the electronic interference effects of neighboringcircuitry (and vice versa, also), the optical fiber technology offers good promise [6,7]. Moreover the optical fiber technology has several other advantages over theconventional PZT-based technology, namely, the electrically passive nature of thehydrophones and lighter and more reliable sensor network which is less prone towater-induced failures, besides their high sensitivity, big dynamic range, and widebandwidth [8–11]. In this context, few efforts have been made in the recent past.Optical fiber-related acoustic pressure-sensing schemes have been reported using asingle-mode fiber (SMF) microphone [12], SMF laser-based hydrophone [13, 14],and fiber Bragg grating hydrophone array [15]. However, the sensitivity is an issuefor most such cases. As compared with conventional SMF, the phase sensitivity issignificantly improved with speciality fibers such as photonic crystal fibers (PCFs),hollow-core photonic bandgap fiber (HC-PBF), and hi-bi polarization-maintainingphotonic crystal fiber (HiBi PM-PCF). Due to significant variations in Young’smodulus because of the air columns running along the fiber length of HC-PBFor PCFs, these systems prove to be good sensors for axial strain applied acousticpressure parameter sensing. Recently, little effort has been put into developingthe PCF-based acousto-optic devices including hollow-core PCF [16–18], solid-core PCF [19, 20], dual-core PCF [21], twin-core PCF [22], and highly birefringentPCFs [23, 24]. Pang and Jin [17] have demonstrated a Michelson interferometer-based system using a 5.7 m fiber to sense the frequencies in a wide range


(40–3000 Hz) at 1550 nm, with phase shift as the function of pressure. Yang et al.[16] have used the similar interferometric technique using an etched and thenpolymer-coated fiber to yield high sensitivity. A Sagnac interferometer has beenused by a group [18] to detect the pressures (0–4000 psi) using a wavelength shift asthe conversion parameter. However, the challenge of a miniaturized system fortypical sonar frequency (especially, low acoustic frequencies between 5 and200 Hz) detection and that too with a good sensitivity is always on the anvil.While the efforts have been few, the system is either quite large (in terms of lengthof fiber), or is fragile (etched fiber, for instance), or has limited sensitivity. Mostly,with the noise in such low-frequency underwater signals, the issue is more challeng-ing, which could probably be addressed by a sensor with good sensitivity and dual-scale (both in terms of power and wavelength change) detection.

In this context, in recent times, various sensors have been demonstrated. A Mach-Zehnder interferometric hydrophone is demonstrated in [25] using polarization-maintaining photonic crystal fiber (PM-PCF). This fiber is spliced between twoSMFs, operated at 1550 nm source. The experimental scheme is based on dualparameter detection for a change in frequency, i.e., both intensity modulationand wavelength shift. The measured data have been compared with the standardhydrophone, and it was found that the PM-PCF sensor shows better response.Figure 3 shows the configuration, and Fig. 4 shows the response of the interferom-eter so formed. A fusion loss of 0.3 dB was found while splicing the sensor fiber(PM-PCF) between two SMFs. This configuration formed the sensor bed. A broad-band laser source of 1550 nm was used, and an Optical Spectrum Analyzer (OSA)was used, both as the source and detector. This sensor bed was passed through a tankfilled with water, and the acoustic frequency (5–200 Hz) was passed. The data wascompared with the reference hydrophone (GRAS, 10CS) data, which was alsoplaced at a fixed distance from the acoustic source. When the acoustic frequencieswere passed through the water, the corresponding interference spectrum wasrecorded at the detector. This was an in-line type of Mach-Zehnder interferometer,having the same physical lengths in both the reference arm and the sensing arm, buthaving different optical path lengths due to the different effective indices of core(ncore) and clad (nclad). Figure 4a shows the comparison data of the power shift(change in power received at the OSA detector, in dBm) as a function of frequency.However, it was seen that the SMF and MMF spliced sensor showed negligiblepower shift, as compared to the PM-PCF sensor, which showed a shift from 0.8 to2.32 dBm. The raw data is shown in Fig. 4b. As shown in the inset of Fig. 4a, thechange in power was accompanied by shift in wavelength of the signal as well. Thewavelength shifted from 50 to 392.8 pm, with the change in acoustic frequency from5 to 200 Hz. The sensitivity of the sensor was found to be 3.15 � 10�3 dBm/Pa(in terms of power) and 5.12 � 10�4 nm/Pa (in terms of wavelength).

The results are mainly understood through the strain-optic effect. Further, thechanges in Young’s modulus of PM-PCF fiber due to the higher axial strain impartedby the acoustic pressure were seen to be responsible to the cause. Using the theory ofbeam bending, the “core modes” and “surface modes” interaction is envisaged and isattributed to the energy transfer between the two fundamental air core modes, which


is possible only through the silica wall. When the pressure is exerted onto thePM-PCF fiber, the thickness of the silica cladding region is reduced, and refractiveindex increases. For higher air-filling ratios, refractive index is larger. Also, as theacoustic pressure is induced on this fiber, the difference between the fast and slowaxes (nx–ny) changes, owing to the phase change in the propagation waves, whichfurther changes the birefringence value, contributing to the sensing phenomenon.

Use of optical fiber approach for low-frequency detections, hence, is a veryaccurate, reliable, and repeatable sensor option; the same can be explored forstructural health monitoring where the complex structures such as bridges andtowers and other building structures could be deployed with these sensors. Thesecould be used to evaluate the vibrational effects on the structures and could bemonitored 24�7 for their health (in terms of wear and tear, cracks, fatigue, and soon).

High-Frequency Detections

These types of sensors are typically related to radar applications. Though it may beslightly debatable if radar-absorbing materials form the part of the discussion onsensors, or not, it is still discussed in this chapter because the material does show

Fig. 3 (a) The experimental setup for the proposed low acoustic frequency detection sensor.(b) The schematic of Mach-Zehnder interferometer and (c) The microscopic image at the interfaceof the SMF and PM-PCF spliced region


property, which is sensitive to the frequency of the incident radiations. Therefore,these are high-frequency detections, when the user is at radar end, and high-frequency absorbers, when the user is at the flight end. In applications with theframe of reference as the “flight,” it is necessary for the flight to remain “hidden”from the radar waves of the enemy radar transmitter, so that the entire aircraft is notidentified in the enemy’s air zone. These types of applications are mostly calledelectromagnetic shielding (EMS). The higher the shielding is, the better is oursecurity in enemy’s air territory. Exactly opposite is our requirement of defensewhen we are on the radar end. The visibility of every other air flight and movingobjects entering in the radar cross section has to be identified. Therefore, the radarsignal (usually in the X band of 8–12 GHz) when is sent by the radar station, if itcomes back and is detected, indicates that the object has reflected the signal back andtherefore there is a flight in the cross section. The time required by the signal to come

Fig. 4 (a) A comparison ofpower shift for SMF, MMF,and PM-PCF sensors as afunction of acousticfrequency. The inset showsacoustic frequency as afunction of wavelength shiftfor PM-PCF. The raw data forPM-PCF are shown in (b)


back to the radar station decides the distance of the object from the radar surveillancestation.

There are many efforts to this effect. One simple way for the aircraft to reduce theelectromagnetic detection is to have reflectors on the aircraft body, which wouldreflect the radiations out of the radar cross section. This is a good option, and fairlygood engineering has to be done for the chassis of the aircraft; and still 100% stealthis not guaranteed. The second option is to have the aircraft completely behave as atransparent object to the entire radiation frequency range and the radiationscompletely pass through, thereby no reflection and hence the stealth application.This is of course extremely difficult and requires a domain of metamaterials forconverting this hypothesis into reality. The third way is to completely absorb theelectromagnetic radiations which are incident upon them. Therefore, there would beno reflection, and hence radar station would not detect the aircraft. It is at this levelthat the high-frequency sensing and absorbing materials come in.

Traditionally various ceramic materials have been used along with polymericsystems such as PVA, PVDF as absorbers. Such materials work on the concept ofskin depth, and the radiations penetrate inside the material and get absorbed there.The thickness of such materials is hence required to be quite large (few mm to cm),and hence the technology gives rise to extremely bulky sheets for electromagneticabsorption. With the advent of nanotechnology, materials of different forms and theircomposites have shown good promise with lightweight and high EMS properties.Carbon-based nanomaterials, along with magnetic nanoparticles, can form a com-posite with a polymer to give polymer nanocomposites which are seen to showextremely good EMS. Not only the coatings are thin, but they also have goodmechanical strength. Such materials find applications in radar EMS and also forother EMS applications which are related to data protection and electromagneticshielding of devices and high-security enclosures. The materials have to be carefullychosen so as to give the effect in a particular frequency range.

The electromagnetic absorption is a function of two primary parameters, namely,the electrical permittivity and magnetic permeability. The material used as e-mabsorber has to have a fine combination of these two properties. While electricalpermittivity is responsible for the e-m radiations to be absorbed through the con-ductivity within the absorbing matrix, the magnetic permeability contributes via thehysteresis loss. The basic idea of the EMS material is to help the e-m energy to getdissipated within the material matrix itself, so that none of the energy is reflected/transmitted. Extremely interesting models have emerged in the recent times,especially with the advent of nanocomposites. Conventionally, iron oxide, nickeloxide, and such other magnetic materials have been used to manipulate the magneticpermeability. Ceramic materials (e.g., alumina) and also polymeric systems (e.g.,PVDF) have been used conventionally as the basic material. However, recently,polymer nanocomposites have been used wherein the concept of percolation thresh-old dominates the EMS phenomenon. Here, a small (and optimized) quantity ofnanomaterial is added in the conventional polymeric matrix, and the material showsgood conducting properties which manipulate the electrical permittivity parameter.Optimal doping is sufficient to make the entire polymer matrix convert into a good


EMS material. The thicknesses have become much lesser (from centimeters tomillimeters), and the frequency response has become much wider. Carbon-basednanomaterials have been at the forefront in this sense. PVA, PVDF, and PTFE are thestandard polymers which have been explored in this context. Additional additivessuch as iron oxide and nickel have been added to improve the shielding efficiency.

Therefore, the future of EMI shielding is ongoing to finally develop lightweightand strong absorption coefficient materials. To attain substantial shielding properties,materials should have suitable dielectric permittivity and losses [26–29]. Higherdielectric loss and latitudinal increase of permittivity lead to better absorptioncoefficient and EM reflection properties, respectively. In the past, numerous studieshave been done on materials such as ceramics and polymers. Traditional ceramics(BaTiO3 and PbTiO3) possess high dielectric constant, but their processing routes,brittleness, and low dielectric strength restricted their usage [30–32]. On the otherhand, polymers having numerous merits like easy processing, flexibility, highdielectric breakdown strength, but their low dielectric constant restricted the appli-cation. At present, polymer nanocomposites showed continuous attention due totheir synergetic effect of polymer matrix and ceramic fillers and thus having superiorproperties than individual [33–35]. Earlier work has been done on polymers such aspoly(vinylidene fluoride) (PVDF) and their copolymers [36] by embedding variousnano-fillers such as barium titanate (BaTiO3) [37], silver [38], and calcium coppertitanate enhancing the dielectric and EMI shielding properties. However, the poten-tial application is still confined due to huge dielectric difference between the polymerand filler, causing limited enhancement. The dielectric and shielding properties ofthe composites depend upon physical properties, interfacial interactions, connectiv-ity among the fillers within the polymeric matrix, and intrinsic conductivity of thefillers [28, 29, 38]. Thus, there is a need to embed semiconducting fillers havingspecial morphology in the polymer matrix in order to achieve suitable dielectricproperties (dielectric loss and conductivity) for EMI shielding applications.

Several efforts have been done on the authors laboratory, with varied materialsand their composites [29, 39, 40]. Various materials like ZnO, PVA, PVDF,graphene, CNTs, graphene oxides, Fe3O4, and porous graphene have been used tostudy the EMS properties of such materials. Properties such as dielectric constants,magnetic permeability, percolation threshold, flexibility, thickness, and mechanicalstrength of these materials have been studied for various types of material compos-ites. Few efforts are summarized below.

Aepuru et al. [39] have demonstrated zinc oxide in polymer composite form,along with poly(vinylidene fluoride) (PVDF) to make nanocomposite thin film. Theyhave observed significant improvement in dielectric properties and electromagneticabsorption with the ZnO fillers. The key results are reproduced in Fig. 5. Negativepermittivity phenomenon has been envisaged, and the results have been projected forgenerating lighter shielding materials, which can be used on aircraft bodies orany radar applications. Their results also predict good mechanical properties ofthe composite film, which is also of utmost important for aerospace applications(refer Fig. 6).


4a

b

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PVDFPVDF-ZnOCommercialPVDF-ZnOSynthesized



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Fig. 5 EMI shieldingeffectiveness (dB) versusfrequency plots of PVDF,PVDF-RZnO (50 wt%)nanocomposite film, andPVDF-CZnO (50 wt%)nanocomposite film withvarying average thickness(a) 105 mm, (b) 210 mm,and (c) 315 mm


Extremely Low (ppm/ ppb) Concentration Detections for CBWDiagnostics

One of the most important applications of sensors from defense perspective is forchemical and biosensing purposes. These are supposed to be miniaturized smartsensors which would detect toxic gases/liquids/moieties in highly sensitive surveil-lance zones. Typically for defense applications, such type of chemical and biologicalmoieties is extremely challenging to sense, either in their vaporized form or theliquid or in their powder forms. Challenge is mainly the concentration of thehazardous moieties. The detection, if is done in the ml concentrations, can be ofless use because the hazard is probably done. However, sensing extremely lowconcentrations (preferably ppm or ppb) of such toxic moieties is extremely usefuland important. Various sensitive zones in the defense estates require this type ofsensor deployment. Similarly, chemical and biological warfare (CBW) diagnostics isa requirement of any country. Therefore, these types of sensors are quite at theforefront.

Biological materials have been sensed using electrical parameters in thepast decades using both conventional electrochemical and microelectrodes methods.Due to recently prevalent micromachining techniques, surface plasmon resonance(SPR) is another parameter which is used to sense bio-entities [41, 42]. Few othermethods are fluorescence [43, 44], electromechanical transduction [45, 46], use ofnanomaterials [47, 48], and other electrochemical tools [49, 50]. Quartz crystalmicrobalance [51] and resonance-based cantilevers [52, 53] are also being exploredfor biosensing in recent times. Though these methods are quite accurate in nature,major limitation is of the nature of process itself. These are mainly laboratory setupsand require a bigger infrastructure for detection. A point-of-care type of detection isnot possible with these methods [54]. Even the preparation of sample requires goodand lengthy chemical procedures, which are good theoretically and also at lab scale,

18

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4

Fig. 6 Stress-strength curvesfor PVDF and PVDF-RZnOnanocomposites underuniaxial tension


but are cumbersome, if used at hospital level or near the patient bed or at the hazard-stricken site. Therefore a lookout for low-cost, on-site sensing device such asdisposable biosensors [55, 56] is always there. Frequency-based biosensors havereceived more attention recently, because of its simplicity and being easy to detect,despite being less accurate along with lesser sensitivity compared to other sensingtechniques. Microcalorimeter falls under this category and is used to develop abiosensor which has a simple detection scheme [57, 58]; however, its fabricationis rather complicated. Conventional sensors also pose standard issues of selectivity,recovery, and detection temperatures. In this context, metamaterial-based sensors arenewly emerging, because of the extraordinary properties, in terms of both frequencyvalues used for detection and the accuracy with which they demonstrate sensing.Metamaterials are an offshoot of the recently explored unconventional sensingprocedures using nanostructured materials, which are being explored due to theirability to “see,” “manipulate,” and “arrange” micromolecules, creating a path formaking nanostructured artificial patterns in the submicron regime.

Metamaterials [59] are artificially engineered electromagnetic materials that are inthe form of arrays of structures, made of metals or any other material with theirperiodicity less than the wavelength of incident electromagnetic (EM) radiation [60,61]. These have been explored for sensing in recent times and have shown extremelygood promise. They can be used in both resonant and nonresonant composite rightleft-handed (CRLH-based) metamaterial types, and various types of sensing param-eters have been studied such as displacement [62, 63], rotation [64], dielectriccharacterization [65, 66], strain sensing [67, 68], mass flow sensor, differentialsensor, temperature sensor, level sensor [69, 70], and so on. The main property ofsuch sensors is that they have extremely small size (much less than wavelength/2)and large Q factor, thereby showing high sensitivity to the parameter which is to besensed. Various applications have already been realized such as sensing ofsolid dielectrics [71], liquids [72], hybrid fuels, hazardous chemicals [66, 73, 74],gas [75, 76], and biomolecules [77, 78]. Therefore, this new category of sensors ishere to stay for the coming few decades and needs proper further evaluation toreplace existing sensors. In this context the authors’ group has put in various effortson sensing of high explosive materials, hybrid fuels, and hazardous gaseous moietiesusing the metamaterial-inspired ring resonator approach [79–81].

Studies on three different high-energy materials, namely, 2-bromo-2-nitropropane-1,3-diol (BNP), bis(1,3-diazidoprop-2-yl)malonate (AM), and bis(1,3-di-azidopropyl)glutarate (AG), have been documented in one of our studies.A complementary split-ring resonator has been fabricated at resonant frequency of4.48 GHz using copper on FR4 substrate. The volume of liquids was varied from 0.5to 3 mL. Prominent and explicit shifts in the transmission resonant frequency andamplitude were seen as a signature of each energetic material. Simulation resultsconcur with the observations. An extremely sensitive, ultrafast, and recoverablesensor is hence described in this work. Figure 7 shows the three different liquidmoieties, which form the base of propellant molecules. As can be seen, they havesimilar structures, and therefore, to discriminate (and, hence, sense) them using achemical approach is very difficult. Figure 8 shows the experimental setup used for


the metamaterial-inspired CSRR structure sensing. It merely comprises of the CSRRstructure, which works as the ground of the sensor, and there is a microstrip line forcarrying the signal. The signal is an RF signal which is sent from a vector networkanalyzer, and the detector in the same VNA collects the response. The CSRR worksas a ground which cuts the electromagnetic lines of the propagating waveform in thetransverse direction, giving rise to a dip in the resonant frequency. Figure 9 showsthis response for various different hazardous propellant moieties, which show unique

O O

2- bromo-2nitropropane-1,3-diol

bis (1,3-diazido prop-2-yl) malonate bis (1,3-diazido prop-2-yl) glutarate

HO OH

Br

NO2

O O O

O O

O

N3N3

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N3 N3

N3N3

Fig. 7 Chemical structures of the high-energy materials

g

Metallization

Substrate

dc

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Lc

ac

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a

Fig. 8 (a) The design of complementary split-ring resonator (CSRR) along with the dimensions;(b) The equivalent circuit of CSRR; (c) The experimental setup. The inset shows enlarged view ofCSRR-based microstrip (bottom right) and enlarged view of CSRR (bottom left)


and different values. Since their resonant frequencies are very unique, their signa-tures with this metamaterial-inspired CSRR structures are also unique.

This kind of sensor which can be used for various sensing environments includesflex-fuel sensing, fuel adulteration, and also other biosensing. One can alsofunctionalize the sensor bed with a nanomaterial which is exclusively sensitive tofew moieties, to make them extremely sensitive and selective. For example, withgold nanoparticles functionalization, the metamaterial sensor could work as a selec-tive hydrogen sulfide gas sensor. This is because gold has special affinity towardsulfur. Hence the sensor will respond to H2S with high selectivity and highestfrequency shift, as compared to any other gaseous moieties such as oxygen orhydrogen or CO. This is a newly upcoming strategy which has been employed toimprove the metamaterial-inspired SRR/CSRRs for selective, fast, and highly sen-sitive sensing.

Another approach in sensing is related to an optical fiber approach. Optical fibers,as are known, have tremendous advantages as compared to conventional sensors,owing to their response via photon, and not via electrons. Less e-m interference andhence high noise immunity are its main advantage. Optical fibers could be used invarious interferometric modes (such as Fabry-Perot, Mach-Zehnder, and so on),along with nanomaterial manipulations, to envisage the sensing. To associate ananomaterial with optical fiber, various techniques are used, such as mirror coating(fiber-end coated with a nanomaterials) or clad manipulation (in which a section ofclad region is etched out and nanomaterial is deposited there) or simple fibermanipulations (subjecting it to stress, strain, and torsions – typically for structuralhealth monitoring/sensing). A speciality fiber such as photonic crystal fibers can alsobe used, and the holes in the fiber could be filled up with a nanomaterial toenvisage sensing of weak fields (electric/magnetic).

In one of our efforts to sense H2S gas [82], nanomaterial-modified optical fibersensor was studied for sensing at room temperature for very low concentrations from1 to 5 ppm. ZnO nanoparticles were embedded into polymethyl methacrylate

Fig. 9 The measured S21response of CSRR loadedmicrostrip for free space andthe three high-energymaterials


(PMMA) matrix and coated on the mirror tip of the single-mode fiber in the Fabry-Perot interferometric configuration to a 1550 nm laser source. The sensor responsewas found to be 1950 p.m. for 5 ppm gas. The detailed study of response of thesensor modified with nanocomposite (and their individual counterparts) suggestedthat the swelling property of PMMA offers enhanced gas adsorption opportunity andoptical transparency of the ZnO nanoparticles exhibit sensing. A real-time sensor,operating at room temperature, is hence projected. This work paves a new avenue forsensing via manipulation of refractive index of the materials, thereby inducingchange in the optical fiber response, in terms of the shift in the interference pattern,both in terms of wavelength and signal power, when subjected to the sensingenvironment. Figure 10 shows the experimental setup of the optical fiber-basedsensing, and Fig. 11 shows the results of the sensor so formed and its comparisonwith the optical sensing data of individual counterparts, namely, PMMA and ZnO.

Owing to the electron configuration of the analyte, it is clear that H2S will interacteasily with the PMMA matrix, followed by negligible interaction between PMMAand O2 (mostly only physisorption). This is because the polar molecule H2S readilygives electron to PMMA (hence inducing substantial swelling), which is furthergiven to the entrapped ZnO. This modifies the electrical and optical properties ofZnO, which induces the change in the effective refractive index of the material.

There are other approaches such as resistive sensors, in which the resistance of thesensor bed changes with their exposure to various gases. Mainly they work in

To detector

3 dBcoupler Gas Chamber

Gas InletSensing region-FPI tip

Vacuum pump

Computer Interface

From source SMF-28

SMF-28

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Fig. 10 Simplified presentation of FPI-OFS setup. Inset represents the FPI tip, which workssensing tip


oxidative or reducing environment, and the sensor changes its resistance dependingupon the ambience of the subjected environment. The typical nanostructured metaloxide materials, which have been used for gas sensing applications, are TiO2 [83],SnO2 [84], ZnO [85], CdO [86], and WO3 [87]. There are an umpteen number ofpapers in this context [88–108], and mainly the interactions are of oxidative/reduc-ing types, which change the resistance of the sample when they are subjected to thesensing environment. Several parameters are taken into account for evaluation of asensor, namely, sensitivity, response and recovery time, detection limit, stability,selectivity, etc. Due to their excellent merits over the other sensors, zinc oxidenanostructures have been one of the most favored materials for various sensingapplications. For example, Kumar et al. have reviewed various ZnO nanostructures,their synthesis techniques, and various parameters like sensitivity and response timefor NO2 gas [109]. Their study shows that the response time of as low as 9 s can beachieved with the help of nanorods, while in case of nanoparticles, it goes to 6 s, andthe recovery time is ~17 s. Another review by A. Wei et al. covers recent

OFS-2Conc.(ppm) OFS-2 OFS-3

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Fig. 11 (a) The spectralresponse of the ZnO-PMMAcomposite before and afterexposure to H2S gas for 60 s;(b) Effect of H2Sconcentration on sensitivity ofOFS-1 for 60 s. Insetrepresents numerical values ofpeak shift with concentrationfor OFS-1, OFS-2, and OFS-3


developments in ZnO sensors, wherein they note that the ZnO films have lowestdetection limit till 60 ppb for NH3 and sensitivity of ~32,000% for ZnO nanowiresfor CO gas sensing [110]. It can be deduced that the response time, recovery time,sensitivity, and stability of the sensors are dependent on many actors includingmorphology, the substrate-analyte chemistry, surface area of the ZnO nanostructure,concentration of the gas, oxygen vacancies, operating temperature, etc.

Though much work has been done in this domain, this approach suffers the issuesof selectivity [109–113]. In this context, a novel approach has been taken up inrecent times, which is of doping the metal oxides with rare elements and noblemetals such as gold, silver, platinum, and palladium. U.T. Nakate et al. [114] havereported the improved LPG sensing using gold (Au)-sensitized ZnO nanorod film.The ZnO nanorods were grown on glass substrate, and the response to LPG gas wasstudied both in only ZnO film and Au-sensitized ZnO at different temperatures.Similar studies were done by the same group, varying the gas (LPG) concentration aswell. The transient gas response and recovery times for sensor were explained underthe light of chemical sensitization and/or electronic sensitization. Figure 12 showsthe response of the sensor to the gaseous environment.

In the electronic mechanism, the action with target gas molecules takes place onthe surface of the additives along with metal oxide surface. These additives changetheir charge state, which results in a variation of the surface barrier height and as aresult change in conductance of the metallic oxide. Au additive as an active catalystcreates more active sites that are believed to be crucial for the enhanced in sensitivity.Surface defects modulated by the chemical and electrical effects of Au also improvethe sensing ability. Au (or any such noble metal) serves as specific adsorption sites todissolve oxygen molecules and adsorb target gas molecules [115, 116]. The additionof Au increases adsorption and enhances dissociation of chemisorbed air oxygenmolecules into oxygen ion species O2 [117]. A phenomenon called “spillover effect”[118, 119] comes into play in the Au-sensitized ZnO nanorods. Oxygen diffuse

Fig. 12 Response curves ofthe ZnO thin film at 623 Kupon exposure of variousconcentration of LPG


increases and capture free electrons from the conduction band of ZnO to create moreoxygen ion species as compared to pure ZnO. This more number of oxygen ionspecies reacts with more number of LPG (refer to Fig. 13). The band diagram used toexplain this sensing is shown typically in Fig. 13.

Low Electric and Magnetic Field Detections

As has been mentioned briefly above, low-field detections are one field which has itsrequirement right from the automotive industries to the shipping industries, toelectronic industries, and to mining applications. Though there has been immenseresearch done and various sensors being realized in the form of weak electric andmagnetic field detections, the requirement of miniaturized and highly sensitivesensors is always there. Moreover, with the advent of micro-/nanoelectronics,

Fig. 13 The cartoon of the process and the schematic band diagram: (a) pure ZnO nanorods;(b) ZnO nanorods sensitized with Au


extremely low values of such fields are required to be detected. Many efforts havebeen done in this regard, such as use of Hall probes, SQUID magnetometry, and soon. However, a new approach is also been studied, which is via using of optical fibersand manipulating their mirror end by a nanomaterial which is sensitive to magneticfields and then studying its magneto-optic response using a specific interferometrictechnique as has been discussed in the section above. The second method is viausing a photonic crystal fiber and filling their holes with magneto fluids andobserving the response to applied fields (electric/magnetic). The results are indeedinteresting and promising too.

Thakur et al. [120] have published their work on development of a magnetic fieldsensor having advantages of both photonic crystal fiber and opto-fluidics, combiningthem on a single platform by infiltrating small amount of Fe3O4 magneto-opto-fluid/nanofluid in cladding holes of polarization-maintaining photonic crystal fiber. It hasbeen demonstrated that magnetic field of few mT can be easily and very welldetected with higher sensitivity of 242 pm/mT. The change in the birefringencevalues has been correlated to the response of nanofluid to applied field. Theadvantages of polarization-maintaining photonic crystal fiber (PM-PCF) and Fe3O4

magnetic nanofluid have been exploited using simple birefringent interferometrictechnique to fabricate the magnetic sensor. Figure 14 shows their experimental setupused and SEM image of PM-PCF fiber. The sensor works on the principle ofbirefringent interferometer and consisted of a piece of PM-PCF connected betweentwo in-line fiber polarizers. An optical sensing analyzer with an inbuilt broadbandsource (1510–1590 nm) has been used to measure the fringe pattern obtained in thetransmission spectrum. Magnetic field was varied from 0 to 80 mT. Figure 15 showsthe response of the sensor to the applied magnetic fields.

Optical Sensing Analyzer

Source

Detector

LinearPolarizer

LinearPolarizer

Solenoid

PM-PCF

Translation Stage

Fig. 14 Experimental setup and SEM image of PM-PCF


Higher magnetic field sensitivity of 242 pm/mT for 0.6 mg/ml concentration ofFe3O4 is reported in this work. Response of iron oxide nanofluid to applied magneticfields may be related to their anisotropic microstructure and alignments in the formof chain-like structures in the fiber microchannels [121, 122]. Such works have beenreported independently by few other research groups, as well. Hence these kinds ofsensors could be categorized as those wherein the sensor fabrication technique iseasy and simple.

This work will lead to design of a miniaturized PCF-based magnetic field sensor,utilizing very small quantity of magnetic nanofluid. Similar efforts are documentedusing cobalt nanoparticles [123].

Challenges in Current Technologies and the Route Ahead

The key building blocks of information security are confidentiality, integrity, andavailability. Confidentiality refers to the idea that only authorized users have correctaccess to assets (in this case means the data transmitted over a wireless network).Sensor-based intelligence gathering has some advantages over more traditionalforms (of intelligence gathering), specifically in the areas of ease of deployment,camouflage, and redundancy. Sensors are small footprint in terms of size and weight,and therefore many can be deployed at once as part of reconnaissance, prior to anengagement. Also, their small size means that sensors can be disguised easily for a

Fig. 15 (Color online) Fringe pattern of PM-PCF filled with 0.6 mg/ml concentrated Fe3O4

nanofluid at different magnetic fields. Inset shows wavelength spacing and birefringence versusmagnetic field


range of environments. Further, sensors can overlap which provides redundancy, ifone or more are rendered inoperative. This dependency on sensors comes at a cost,however. Potok et al. [124] point out that in such systems, information must betransmitted securely under often suboptimal network conditions; otherwise theirvalue is severely negated.

As is said, it is practically impossible to construct a truly secure informationsystem. Communications are secure if transmitted messages can be neither affectednor understood by an adversary, likewise, information operations are secure ifinformation cannot be damaged, destroyed, or acquired by an adversary. They goon to define software challenges for a future combat system including (but notlimited to) network security and accessibility, fault tolerance, and informationanalysis and summary of large data streams from the network. Further, Shostackand Stewart [125] claim that most software is insecure. This could be because, asWysopal et al. [126] have observed, security requirements are often omitted fromrequirements specifications altogether. This has been noted as being particularlyproblematic in other safety-critical domains such as automotive control software aswell [127]. In terms of the problem domain (military operations), wireless sensors ofvarious types can be distributed on ground before a battle while being connected toautonomous software agents in a multi-agent system to give an on-field tacticaladvantage, provided that the communications between the sensors cannot be sub-verted. A public key infrastructure is an obvious solution to the integrity problem;however, issues of secure storage for the private key and over-the-air transmission ofeither public or private keys will still prove problematic. The issue of key manage-ment is perhaps further complicated by the ever-decreasing cost of the hardwarerequired to conduct a brute-force attack. For example, a multi-TeraFLOP GPGPUcluster can be purchased for as little as AUD$10,000. Another area of concern iswhether the agents themselves can be subverted. As noted above, while truly securesoftware is almost impossible to create, it may be that security-oriented softwaredevelopment methods that place security requirements at the forefront of all stages ofthe development lifecycle will reduce or eliminate vulnerabilities in this area.

Conclusion

In conclusion, a literature survey of different kinds of sensors for various applica-tions pertaining to defense sector is done. The main domains cover the chemical andbiological warfare diagnostics, radar electromagnetic shielding applications, low-and high-field electric/magnetic field sensing, and low-frequency (SONAR) sensing.Various approaches to envisage these sensors are discussed, such as optical fiberapproach, metamaterial-inspired antenna approach, nanomaterials resistive-sensorapproach, and polymer nanocomposite films approach for electromagnetic absorp-tion/reflectance/transmission. These are all new and upcoming techniques used tocombat eavesdropping using miniaturized sensing elements. The miniaturization isachieved via use of nanomaterials which provide more sensing area in lesservolumes and metamaterial approaches, which shrink the antenna sizes to sizes lesser


than lambda/2. Though there is a long way to go to develop these sensors using anon-silicon fabrication approach, this is sure to take up the commercial market incoming years. Sensors development is going to be one of the most promising andimpending technologies, which would connect to various IOT devices to make themsmarter, smaller, sensitive, selective, and faster.

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